This invention relates to improvements in shrouded fan assemblies, for example in cooling fan assemblies used to move air through automotive radiators.
In many applications, the space available for mounting a fan is confined, requiring the use of a small fan, while the fanning capacity required dictates the use of a large fan.
For example, the automobile space engine compartment is being reduced in size with the reduction in the overall size of automobiles, leaving less and less room for the mounting of a cooling fan. Furthermore, various control devices and options such as air conditioning reduce the room available for mounting a fan, while at the same time they increase the demand for cooling capacity which the fan must meet.
It is therefore increasingly important to maximize the fanning capacity of a fan of a given size, and fan efficiency has therefore become a crucial design consideration.
Fan efficiency losses occur in large part due to undesirable airflow around the tips of fan blades in the gap which exists between the fan blade tips and a wall which surrounds the rotating fan. The undesirable airflow can generally be described as backflow of air from the exhaust side to the intake side of the fan, recirculation of air in vortex patterns inside the gap, and localized backflow over each fan blade tip from the pressure side to the suction side of the blade. These flows produce no useful cooling air flow, cause fan noise, and lower fan efficiency.
Various designs have been proposed for increasing fan efficiency by reducing these flows. The designs generally involve an interruption or decrease in the size of the gap. Losses have been reduced by fan designs that work with a stationary shroud, or by modifications of the fan blades such as bent back tips, winglets and guide vanes.
One approach of the prior art has been to mount a rotating annular shroud to the fan blade tips and then to position this rotating shroud against a stationary shroud in a labyrinthian or some other configuration, as shown in U.S. Pat. No. 3,842,902. While effective at reducing the undesirable air flow, this design suffers from certain disadvantages. Provisions must be made for mounting the rotating shroud onto the blade tips and for assuring that the shroud will remain mounted during rotation at high speed. Care must be taken that the rotating shroud and the blade assembly that the rotating shroud is mounted to are true, so that the fan does not undergo excessive oscillations during operation. Also, fan blade vibration causes stresses to develop at the areas of attachment of the rotating shroud to the blades. These design considerations increase the cost of implementing the rotating shroud approach.